Search

Purpose: The paper reports on progress in physics of radiationless transitions and new Auger spectra of 125I and 124I. We report progress in Monte Carlo track structure simulation of low energy electrons comprising majority electrons released in decay most Auger emitters. Materials and methods: The input data for electron capture (EC) and internal conversion(IC) were obtained from various physics data libraries. Monte Carlo technique was used for the simulation of Auger electron spectra. Similarly, electron tracks were generated using Monte Carlo track structure methods. Results: Data are presented for the EC, IC and binding energy (BE) of radionuclides 124I and 125I. For each of the radionuclides 125I and 124I some examples of electron spectra of individual decays are given. Because most Auger electrons are low energy and short range, data and a short discussion are presented on recent Monte Carlo track structure development in condensed media and their accuracy. Conclusions: Accuracy of electron spectra calculated in the decay of electron shower by Auger emitting radionuclides depends on availability of accurate physics data. There are many gaps in these libraries and there is a need for detailed comparison between analytical method and Monte Carlo calculations to refine the method of calculations. On simulation of electron tracks, although improved models for sub-keV electron interaction cross sections for liquid water are now available, more experimental data are needed for benchmarking. In addition, it is desirable to make data and programs for calculations of Auger spectra available online for use by students and researchers. [ABSTRACT FROM AUTHOR]

Purpose: To study the energy deposition by low-energy electrons in submicron tissue-equivalent targets by comparing two widely used methodologies, namely, the continuous-slowing-down-approximation (CSDA) convolution integral and the Monte Carlo (MC) simulation. Methods: An MC track-structure code that simulates collision-by-collision the complete slowing down process is used to calculate the energy deposition in spherical volumes of unit density water medium. Comparisons are made with calculations based on the CSDA convolution integral using both empirical and MC-based range-energy analytic formulae. Results: We present self-irradiation absorbed fractions and S-factors for monoenergetic electrons of initial energies from 0.1-10 keV distributed uniformly in spheres of 5, 10, 50, 100, 500, and 1000 nm radius. The MC and CSDA results were found, in some cases, to differ by a factor of 2 or more; differences generally increase with decreasing sphere size. Contrary to high energies, the uncertainties associated with the straight-ahead approximation implicit in the CSDA calculations are of the same order as those related to straggling and δ-ray effects. Conclusion: The use of the CSDA methodology may be unsuitable for the sub-micron scale where a more realistic description of electron transport becomes important. [ABSTRACT FROM AUTHOR]

Abstract: Optimizing targeted radionuclide therapy for patients with circulating malignant cells (e.g. blood-related cancers) or a micrometastatic spread requires quantification of various dosimetric parameters at the single-cell level. We present results on the energy deposition of monoenergetic electrons of initial energy from 100eV to 20keV – relevant to Auger emitting radionuclides – distributed either uniformly or at the surface of spherical volumes of radii from 10nm to 1μm which correspond to critical sub-cellular targets. Calculations have been carried out by our detailed-history Monte Carlo (MC) code which simulates event-by-event the complete slowing down (to 1Ry) of both the primary and all subsequent generations of electrons, as well as, by the continuous-slowing-down-approximation (CSDA) using analytic range-energy relationships. The latter method has been adopted by the MIRD committee of the Society of Nuclear Medicine for dosimetry at the cellular level (>1μm). Differences between the MC and CSDA results are up to ∼50% and are expected to be even larger at higher energies and/or smaller volumes. They are attributed to the deficiencies of the CSDA method associated with the neglect of straggling and δ-ray transport. The results are particularly relevant to targeted radiotherapy at the genome level by Auger emitters. [Copyright &y& Elsevier]

Abstract: Our MC4 Monte Carlo code which simulates in an event-by-event mode the electronic losses and slowing down process of protons and all their secondary electrons in water is used to obtain dosimetric distributions of interest to the biophysical modelling of proton beams at the submicron scale. Energy loss scattering models constructed from the oscillator strength formalism of Bethe as well as Ritchie’s dielectric approach have been implemented in order to explore the influence of non-linear density effects induced by the liquid-like cellular environment. We present results on energy deposition and its straggling for nanometer spherical volumes relevant to DNA- and chromatin-size targets. The influence of the impact geometry and the size of the volume are examined. [Copyright &y& Elsevier]

Abstract: Distributions of relevance to proton beam profiling in soft biological matter are obtained by our Monte Carlo track structure code (MC4), which simulates in an event-by-event mode the electronic losses of light ions and all their secondary electrons in water. We present results on electronic energy density and clustering of interaction events around MeV proton tracks which are associated with important “local track” parameters. The detailed character of the simulation based on single-collision cross sections for all the elementary interaction processes allows a probing resolution of the order of nanometers. Since soft biological tissue exists in a liquid/solid like state we compare results obtained by both the oscillator strength properties of the water molecule and the dielectric response of liquid water. Their differences in terms of the nanometric pattern of proton tracks is examined. [Copyright &y& Elsevier]

Abstract: Monte Carlo track-structure simulation provides a near accurate description of the passage of charge particle in water as a surrogate for biological tissue. Radiation transport codes and Monte Carlo track-structure codes are widely used in radiation biophysics, dosimetry and microdosimetry, clinical radiotherapy, in space radiation program and accelerator design and research. Over the past decade the number of Monte Carlo track codes simulating a variety of different types of radiations has increased rapidly. In this paper we provide a review of recent progress in the development of particle track simulation for electron, low-energy light ions and finally the recent model development for the low-energy electron cross-sections in liquid water. [Copyright &y& Elsevier]

In radiation targeted therapy and genetic risk estimation of low dose radiation protection there is a crucial need for full description of DNA damage response and repair (DDR) leading to cell death and cell mutation. We propose such a description can be arrived through realistic track-structure simulations together with mechanistic mathematical formulation of DDR and the availability of experimental data for testing the proof of principle. In this paper we review briefly first the state of the art in DNA damage and repair, and then the recent advances in the physics of track structure which represents an essential tool in radiation biophysics. [ABSTRACT FROM AUTHOR]